Process for improving parts formed by powder metallurgy by addition of spiegeleisen to metal powders



United States Patent C PROCESS FOR IMPROVING PARTS FORMED BY POWDER METALLURGY BY ADDITION OF SPIEGELEISEN TO METAL POWDERS Application October 16, 1956 Serial No. 616,121

5 Claims. (Cl. 75-214) No Drawing.

The present invention relates to a process of improving the tensile strength of parts formed by powder metallurgy by adding spiegeleisen thereto. More particularly the present invention involves the preparation of parts by powder metallurgy, wherein the parts contain carbon to a greater or less extent, either that amount of carbon introduced with the spiegeleisen or a total amount of carbon including the part introduced with the spiegeleisen and an additional amount separately introduced as such.

It has been known for some years to make various parts by powder metallurgy using iron or materials consisting essentially of iron and including in addition thereto some one or more materials such as are conventionally present in many known alloy steels. Particularly, it is known to introduce some carbon along with iron powder in the making of iron parts by powder metallurgy, so as to enhance the tensile strength thereof and to produce parts more nearly simulating steel than parts which are of pure iron. It is also quite conventional to make parts of mixtures and/or alloys of metals, as for example, copper and iron, wherein the copper will alloy with the iron to a minor extent only and wherein it is believed that a mixture exists to the extent beyond that at which the metals can alloy together. Various alloy mixtures have also been resorted to for the purpose of increasing the strength of parts made by powder metallurgy. In many instances the metals which are required to be added to the iron in order to secure increased strength are themselves quite costly, at least in the proportions in which they are required to be present in order to obtain desired increases in the strength of the finished parts. Carbon in the form of graphite is relatively inexpensive as contrasted with the cost of metallic alloying ingredients, so that if the desired increase in strength can be accomplished by adding carbon, then a desirable combination is attained. It is found, however, that the assimilation of carbon to produce articles having the desired high tensile strength is often something less than that wished for, or perhaps for some other reason, tensile strengths of desired high values cannot be obtained at a minimum cost.

It has also been proposed to add manganese in several known forms to iron powder for increasing the strength of the articles made therefrom.

The present invention is based upon a novel discovery that when manganese, in the form of spiegeleisen is added to iron powder, exceptionally desirable results are obtained, even as compared with the addition of corresponding amounts of manganese in other forms. These desirable results are particularly noteworthy and useful in instances where more or less carbon is present in the material as fabricated. It seems likely, that the addition of spiegeleisen assists in some unknown manner in the assimilation of carbon, so that articles formed in this manner have surprisingly high tensile strengths as compared with articles formed in accordance with any one of a number of prior art practices, which are essentially similar, except that they do not use spiegeleisen as such,

but may use certain of the constituent elements thereof. The theories as to why these novel and surprising results are obtained have not yet been formulated. It is known, however, that the desirable results of the present invention may be obtained in a certain and reproducible manner if the teachings of this invention are followed.

Turning now to the essentials of the present invention, it is found that the material to which the spiegeleisen is added in accordance therewith is iron and material consisting essentially of iron. By this is meant that the material present is principally and predominantly iron, but may have some one or more other materials therewith, which, if present, do not substantially affect or change the general qualitative effect of the addition of spiegeleisen in accordance with the present invention. That is to say, a material consisting solely of iron in powder form and to which spiegeleisen is added will, when made into finished parts, have a substantially higher tensile strength than parts made of the same iron (alone) without spiegeleisen. By the same token, an iron alloy part may have a tensile strength (without spiegeleisen) substantially greater than that of a pure iron part. Even here, however, this greater tensile strength will be further enhanced to a substantial degree by the addition of spiegeleisen thereto. This is true even though it may be that the addition of spiegeleisen to a pure iron powder will give a part having a tensile strength less than that of an iron alloy part without spiegeleisen. In each instance, however, the tensile strength without spiegeleisen is respectively substantially enhanced by the addition of spiegeleisen. Thus, it may be said from the point of view of the present invention, the metal powders, which are in each instance mostly iron, usually over react qualitatively in the same way as pure iron in that they are all substantially improved by the addition of spiegeleisen. Thus, it is considered from the point of view of the present invention that all such materials may be classified as consisting essentially of iron, which is the manner in which all such similar materials are classified in accordance with the present invention both in this description and in the appended claims.

The particle size for the metal powders used in the process of this invention is not particularly critical. In most commercial specifications, metal powder used for the formation of parts by powder metallurgy must be at least as small as mesh (Tyler standard screen). Such powders are adequate in accordance with this invention. In general, the spiegeleisen to be used is preferably about the same particle size or smaller than the metal powder with which it is used; although again this is not critical, and the particle size should be sufficiently small so that good mixing is attained. Furthermore, the mixed powder should be small enough in particle size as to be not unduly abrasive. It has been found that l00 mesh powder is satisfactory and operative; that -200 mesh powder may also be used; and further that -325 mesh powder is fully operative and is effective from the point of view of obtaining desired tensile strengths as herein set out.

The mixing of the spiegeleisen with the other metal powder or powders with which it is to be used may be effected in any desired manner, these powders usually being mixed on a dry basis to form as homogeneous a mixture as can be done in a practical manner and at a reasonable expense. No special type of apparatus is needed; but any apparatus conventionally usable for making dry mixtures of two or more types of powders may be used.

spiegeleisen itself has a composition in the general range of about 17-21% manganese, about 36% carbon, and the remainder principally iron.

The particular type spiegeleisen which was used in the test hereinafter described will be set out in connection with the examples which follow:

EXAMPLE I The purpose of this example is to show the substantial improvement which is always attained by the use of the present invention as contrasted with the prior art wherein no spiegeleisen was used. In each test given in this example about 2% spiegeleisen was added to different types of iron powder as hereinafter set forth as to each test. The spiegeleisen used in this test contained 20.65% manganese, 3.55% carbon and the remainder mostly iron. In each instance the iron powders used had a particle size of 100 mesh. In each instance the powders were blended with l /2% of graphite (carbon) 1% of zinc stearate (as a mold lubricant) and 2% of -325 mesh spiegeleisen having the composition aforesaid. Following the mixing of powders with or without spiegeleisen respectively as hereinafter set out, the metal powders were compressed at a pressure suflicient to give a green pressed blank having a density of about 6.0 grams per cc. These blanks were then sintered at 2030 degrees F. for about hour in a dry exo-gas which had a composition of 12% carbon monoxide, 17% of hydrogen, 71% of nitrogen and a relatively small amount (not measured) of water vapor.

In a first pair of these tests, an iron powder was used which was the product of the process set out in the Crowley Patent No. 2,744,002, issued May 1, 1956. With spiegeleisen, parts made as aforesaid had a tensile strength of 24,200 p.s.i., while with 2% spiegeleisen such parts had a tensile strength of 32,000 p.s.i.

In a second pair of tests using another type of iron powder, parts having 0% spiegeleisen had a tensile strength of 24,400 p.s.i., while parts having 2% spiegeleisen incorporated therewith had a tensile strength of 38,600 p.s.i.

In a third pair of tests employing another commercial type of iron powder known as Swedish Sponge, parts having no spiegeleisen had a tensile strength of 18,800 p.s.i.; whereas parts with 2% spiegeleisen incorporated therewith had a tensile strength of 30,800 p.s.i.

In a fourth pair of tests using an iron alloy powder, including not only iron, but also about 1% nickel and about A molybdenum, and which was made in accordance with the copending application of Reed et al., Serial No. 577,191, filed April 10, 1956, for Process of Making Parts by Powder Metallurgy and Preparing a Powder for Use Therein, now Patent No. 2,799,570, issued July 16, 1957. Parts made of this powder having no spiegeleisen incorporated therewith had a tensile strength of 25,000 p.s.i.; while similar parts having 2% spiegeleisen added as aforesaid had a tensile strength of 39,000 p.s.i.

EXAMPLE H Table 1.Variation in amount of spiegeleisen added Percent spiegeleisen: Tensile strength From the data in Table 1 it will be apparent that substantial improvement in tensile strength is attained with about 4 1% spiegeleisen; that the maximum improvement is attained with about 2% spiegeleisen; and that further additions of spiegeleisen over and above about 2% serve little or no purpose. Thus, while it is not prohibitive from the operative point of view that more than about 2% spiegeleiscn be used, spiegeleisen at least in the fine powder form in which it is used is so much more expensive than are the other metallic powder ingredients, there is a practical economic limit on the amount of spiegeleisen used at about 2%. The preferred content of spiegeleisen in accordance with this invention, therefore, is about 2%.

EXAMPLE III This example sets out the results of tests made to show the effect of different sintering temperatures on the tensile strengths of parts made with and without spiegeleisen respectively. In this example the same type iron powder was used as in the first portion of Example I and in Example II and the parts were made in accordance with the particular teachings set out in Example I.

Parts containing zero percent spiegeleisen were sintering respectively at 2030 degrees F. and 2070 degrees F., these parts having tensile strengths respectively of 24,200 and 22,600 p.s.i. In another pair of tests, the parts having 2% spiegeleisen in each instance in addition to the same basic composition were sintered again at 2030 degrees F. and 2070 degrees F. respectively. These parts had respective tensile strengths of 32,000 p.s.i. and 32,200 p.s.1.

Thus, while it is clear that the presence of spiegeleisen in each instance effected a substantial increase in tensile strength, it would appear from these tests that variations in sintering temperature had little effect on the tensile strength.

EXAMPLE IV This example sets out a series of tests attempting to determine the reason for the very desirable results of the use of spiegeleisen in accordance with the present invention. It was thought at the outset that the highly desired results obtained from the addition of spiegeleisen could be attributed to the presence of manganese in the final product. In order to determine if this was so, a series of tests were run, all with the same iron powder and with a sufiicient amount of dififerent sources of manganese in admixture with the iron powder so as to give a manganese content in each instance of 0.4% in the final product. Except as herein particularly stated, each powder combination was made up with 1 /2% graphite and other additions as particularly set forth in Example I.

This example includes the following tests: (a) with standard ferro-manganese (74-76% Mn, 6.5%7.5% C) as a source of manganese the parts had a tensile strength of 18,800 p.s.i.; (b) with low carbon ferro-manganese (85-90% Mn, less than 0.5% C) the parts had a tensile strength of 20,000 p.s.i.; (c) with medium carbon ferromanganese (-85% Mn, l.25%-1.5% C) the parts had a tensile strength of 20,000 p.s.i.; (d) with pure manganese, parts had a tensile strength of 19,500 p.s.i.; and (2) with spiegeleisen (20.65% Mn, 3.55% C) the parts had a tensile strength of 32,000 p.s.i.

It may therefore be reasonably concluded from the tests of this example that something substantially other or in addition to the presence of manganese must account for the exceptionally fine results of the use of spiegeleisen according to this invention. Furthermore, as the carbon content was substantially the same in several of these instances, the mere presence of a given amount of carbon per se cannot explain the results of the operation of the present process. Just what is the explanation of these desired results of this invention is not known. It is known, however, that these results may 'be attained again and again by following the teachings of this invention.

EXAMPLE v The purpose of this example is to set out certain tests and to discuss the effect on tensile strength of the final articles of different amounts of carbon therein. It must be recognized that whenever spiegeleisen is used, some carbon is necessarily added, as spiegeleisen itself contains carbon.

It has been found that when a particular type of iron powder (different from that used in other examples) was used, parts made from this iron powder alone (no spiegeleisen and no carbon) had a tensile strength of 13,500 p.s.i.; whereas other parts made from this same iron powder with 2% spiegeleisen added, but with no additional carbon added, had a tensile strength of 19,300 p.s.i. These differences are believed to be greater in proportionate amount than could reasonably be explained by the amounts of either carbon or manganese added in the form of spiegeleisen.

It has also been found as a result of a large number of tests that maximum tensile strengths are attained with a carbon content of about 0.85% in the final product, tensile strengths building up gradually to a maximum as the carbon content is raised up to this value and decreasing gradually as the carbon content is further raised beyond this value. It is thus desirable to be able to predetermine the amount of carbon to be contained in the final product. As stated above, some carbon is added when spiegeleisen is used, as carbon is a constituent of spiegeleisen. This amount must, of course, be taken into account. However, in order to obtain a desired high tensile strength, it is usually necessary to add some additional carbon. This is ordinarily added when the metal powders to be used are blended together, the added carbon being preferably in the form of powdered graphite.

In order, however, to obtain a good predetermined selected amount of carbon in the final product, it is necessary usually to add somewhat more carbon than that theoretically required to provide the desired carbon content. The reason for this is that most iron powders exhibit what is termed in the industry a hydrogen loss. This is explained in the following manner: Most iron powder which is commercially available contains some small amount of iron in some oxide form. If this commercial iron powder, containing some iron in oxide form, be heated in pure hydrogen, the iron oxide present will be reduced to metallic iron; and as such, the sample thus treated will lose weight due to the abstraction of oxygen therefrom. This loss in weight, expressed as a percentage of the starting weight, is termed hydrogen loss and corresponds in effect to the Weight of the oxygen present as an oxide of iron.

When carbon is added to such a powder and the mixed powder is pressed and then siutered, a part of the carbon reacts with the oxygen present at the relatively high sintering temperature to yield some gaseous oxide of carbon. This not only eliminates the oxygen, but also eliminates that portion of the carbon which combined with the oxygen and was dissipated as a gas in the sintering atmosphere. It is necessary, therefore, to take into account the amount of oxygen present, initially expressed as hydrogen loss, in determining how much carbon need be added to a commercial iron powder in order to produce a product having a given or predetermined carbon content. Further, and as a particular refinement, inasmuch as oxygen has an atomic weight of 16 and as the atomic weight of carbon is 12, and assuming that the gaseous product during the sintering is carbon monoxide (CO), then it will be necessary to add carbon corresponding to about /1 of the hydrogen loss when the latter is expressed in terms of the weight of oxygen present. Actual tests have shown that this practical manner of calculation closely approximates the results desired in attaining a predetermined carbon content in the final product.

While there is herein disclosed a process-for the attainment of superior results in accordance with this invention and the limits of certain variables connected with the process are stated, other equivalents will suggest themselves to those skilled in the art from the foregoing disclosure. We do not wish tobe limited, therefore, except by scope of the appended claims which are to be construed validly as broadly as the state of the prior art permits.

What is claimed is:

1. The process of improving the tensile strength of parts formed by powder metallurgy, comprising the steps of mixing (1) a metallic powder having a particle size suitable for powder metallurgy and consisting essentially of iron, with (2) spiegeleisen in powder form and having a particle size at least as small as the particle size of said metallic powder, said spiegeleisen consisting essentially of about 17%21% manganese, about 36% carbon, and the balance iron, said spiegeleisen being present in an amount sufiicient substantially to enhance the tensile strength of the finished parts and not over about 3% by weight; thereafter press-molding the parts to be made from the mixed powders aforesaid to form green pressed blanks, and sintering said green pressed blanks.

2. The process according to claim 1, in which the spiegeleisen is present in an amount of about 2% by weight.

3. The process of improving the tensile strength of parts formed by powder metallurgy and containing carbon, comprising the steps of mixing (1) a metallic powder having a particle size suitable for powder metallurgy and consisting essentially of iron, (2) spiegeleisen in powder form and having a particle size at least as small as the particle size of said metallic powder, said spiegeleisen consisting essentially of about 17%21% manganese, about 3-6% carbon, and the balance iron, said spiegeleisen being present in an amount sufficient substantially to enhance the tensile strength of the finished parts and not over about 3% by weight, and (3) a selected amount of carbon in powder form; thereafter press-molding the parts to be made from the mixed powders aforesaid to form green pressed blanks, and sintering said green pressed blanks.

4. The process in accordance with claim 3, in which the amount of powdered carbon (3) added as aforesaid in the making of a part which is to have a predetermined selected carbon content is that theoretically required to provide said predetermined carbon content, plus an added amount which is predetermined in accordance with the hydrogen loss value of the iron powder (1) aforesaid.

5. The process in accordance with claim 4, in which the additional amount of carbon added as aforesaid is sufiicient to provide about 0.85 carbon in the finished part.

References Cited in the file of this patent UNITED STATES PATENTS 2,152,006 Welch Mar. 28, 1939 2,175,850 Patterson et al. Oct. 10, 1939 2,289,897 Balke et a1. July 14, 1942 2,291,685 Brassert Aug. 4, 1942 OTHER REFERENCES Goetzel: Treatise on Powder Metallurgy, vol. 1,

page 207, 1949. 

1. THE PROCESS OF IMPROVING THE TENSIL STRENGTH OF PARTS FORMED BY POWDER METALLURGH, COMPRISING THE STEPS OF MIXING (1) A METALLIC POWDER HAVING A PARTICLE SIZE SUITABLE FOR POWDER METALLURGY AND CONSISTING ESSENTIALLY OF ION, WITH (2) SPIEGELEISEN IN POWDER FORM AND HAVING A PARTICLE SIZE AT LEAST AS SMALL AS THE PARTICLE SIZE OF SAID METALLIC POWDER, SAID SPIEGELEISEN CONSISTING ESSENTIALLY OF ABOUT 17%-21% MANAGANESE, ABOUT 3-6% CARBON, AND THE BALANCE ION, SAID SPIEGELEISEN BEING PRESENT IN AN AMOUNT SUFFICIENT SUBSTANTIALLY TO ENHANCE THE TENSILE STRENGTH OF THE FINISHED PARTS AND NOT OVER ABOUT 3% BY WEIGHT; THEREAFTER PRESS-MOLDING THE PARTS TO BE MADE FROM THE MIXED POWDER-AFORESAID TO FORM GREEN PRESSED BLANKS, AND SINTERING SAID GREEN PRESSED BLANKS. 